Syrian security forces have reportedly killed dozens of people in the biggest day of protests in five weeks of demonstrations against the regime of President Bashar al-Assad.

The AP quoted witnesses and a human rights groups who had seen the dead and injured. Ammar Qurabi, the head of Syria's National Organization for Human Rights, said the death toll had reached 49 by nightfall and at least 20 more people were unaccounted for.

The reports could not be verified because international journalists have been expelled from Syria.

Eyewitness accounts described tens of thousands of people protesting across the country on April 22 -- in the Damascus suburb of Douma, the central cities of Hama and Homs, Latakia and Banias on the coast, the northern cities of Raqqa and Idiib, the northeastern Kurdish region, and the southern province of Daraa.

The White House said it "deplored the use of violence" and that it was monitoring the situation closely. Presidential spokesman Jay Carney said the White House called on "all sides to cease and desist from the use of violence."

The deadly crackdown came a day after Assad introduced concessions meant to quell the tide of discontent against his rule but which opponents derided as inadequate.

They included the end to a notorious state of emergency that has existed for nearly half a century and also conceded people's right to seek permission to stage peaceful protests. Human rights groups had said the government's response to the April 22 gatherings would be a test of the reforms' authenticity.

In Jasim, a city in the governerate of Daraa in the south of the country, a large crowd carrying Syrian banners chanted antigovernment slogans, including, "The people want to overthrow the regime."

The latest clashes followed reports of heavy security-force deployment overnight as the authorities prepared to meet the opposition's call for a nationwide day of protest.

At least 220 people have been killed, according to human rights groups, in five weeks of demonstrations against what is considered one of the Middle East's most autocratic states.

Assad had prefaced his reforms by declaring last week that there would be no more "excuse" for demonstrations once the state of emergency had been lifted, a central demand of the protests.

However, opponents signaled that this was no longer sufficient. Ahead of the April 22 gatherings, an umbrella group representing the protesters issued a statement demanding a "rapid reformulation of our national institutions."

The statement -- issued in the name of "Syrian local organizing committees" -- demanded an end to torture, killings, arrest, and violence against demonstrators as well as three days of state-sanctioned mourning for those killed so far. It also called for the release of all political prisoners and an independent inquiry into the death of protesters.

Most radical of all, it called for drastic constitutional reform that would limit presidents to two terms. Assad has been president for 11 years, having inherited the post on the death in 2000 of his father, Hafez al-Assad, who ruled Syria for the previous 30 years.